1. Field of the Invention
The invention relates to a live insertion circuit which enables live insertion of only packages in need of maintenance while power is supplied to a maintenance system for maintaining a power station or the like when commercial power service is cut.
2. Description of the Prior Art
FIG. 14 is a diagram illustrating how an insertion package is inserted into a mounting unit. In the figure, reference numeral 11 represents a mounting unit, and 13, 14 and 15 insertion packages to be inserted into the mounting unit 11. Examples of the insertion package include hard disks and logic cards. FIG. 15 shows a prior art live insertion circuit described in "Physical Design Technologies for Network Node Processors", NTT's R&D Technical Bulletin, Vol. 40, No. 10, 1991. In FIG. 15, reference numerals 6 and 7 represent power line terminals, 5 a ground line terminal, and 8 a signal line terminal. These terminals differ in pin length: the power line terminal 6 and the ground line terminal 5 are long pins, the power line terminal 7 a medium pin, and the signal line terminal 8 a short pin. This live insertion circuit is present on the insertion package 13 which is inserted into the mounting unit 11, whereby power is supplied to each circuit within the insertion package 13 through the mounting unit 11 and the live insertion circuit shown in FIG. 15. A load 4 indicates a power load on the entire insertion package 13. In FIG. 15, reference numeral 21 indicates an inductor, 2 a diode for short-circuiting a reverse voltage which is generated in the inductor 21 when the package is pulled out, and 3 a bypass capacitor. FIG. 16 and FIG. 17 show changes in electric current with the elapse of time when the package is inserted. In other words, these figures show the amount of current supplied to the insertion packages 13, 14, 15 in the mounting unit 11 from a power source when a value of the inductor 21 is too large or too small. In FIGS. 16 and 17, reference numerals 31 and 41 indicate the times when the long pin terminals 5 and 6 are connected, and 32 and 42 the times when the short pin terminal 8 is connected. The power terminal 6 is referred to as "long-pin terminal", the power terminal 7 "medium-pin terminal", the ground line terminal 5 "long-pin terminal", and the signal line terminal 8 "short-pin terminal" hereinafter.
A description is subsequently given of the operation of the live insertion circuit. It is supposed that the insertion packages 14 and 15 are already inserted into the mounting unit 1 and put into operation. At this time, suppose that there arises the need to add another insertion package 13 in order to increase capacity and improve performance. The insertion package 13 is inserted into the mounting unit while the insertion packages 14 and 15 are kept in operation. In case of live insertion of the insertion package 13, the terminals may be damaged by the generation of a large surge current caused by the charging current of the bypass capacitor 3 and a current flow into the load, or the power voltage of the mounting unit 11 may be temporarily lowered by quick discharge of a charge by a surge current, resulting in the malfunction of the insertion packages 14 and 15 in operation.
To prevent the malfunction of the insertion packages, the live insertion circuit of the prior art is designed to eliminate sudden changes in power current. In other words, to insert the insertion package 13 into the mounting unit 11, the long-pin terminals 5, 6 of the live insertion circuit shown in FIG. 15 are first connected to the mounting unit 11. Thereby, a current begins to flow into the insertion package 13 through the inductor 21. However, due to the presence of the inductor 21, a surge current does not flow into the bypass capacitor 3 and the load 4, and the insertion package is gradually applied by a current and charged. Thereafter, when the medium-pin terminal 7 is connected, a current flows into the bypass capacitor 3 bypassing the inductor 21, but the bypass capacitor 3 is charged to a certain extent with a charge supplied from the long-pin terminal 6, and a potential thereof is raised. In consequence, a surge current is smaller than when the insertion package 13 is inserted into the mounting unit 11 without the long-pin terminals 5 and 6. The, after the power system of the insertion package 13 is stabilized, the short-pin terminal 8 is connected.
As described above, the live insertion circuit of the prior art employs such a countermeasure against an error caused by a surge current, but it is not adequate. That is, when the insertion package is inserted, if a value of the inductor 21 is too large, as shown in FIG. 16, at the time 31 when the long-pin terminals are connected, there is no sudden change in current, but a time constant is high, and at the time 32 when the medium-pin terminal is connected, a surge current flows into the bypass capacitor 3 since the bypass capacitor 3 is not fully charged. And, if a value of the inductor 21 is too small, as shown in FIG. 17, a surge current flows in at the time 41 when the long-pin terminals are connected. The prevention of a surge current is not perfect at non-continuous points of impedance where pin terminals are put into contact or separated. Therefore, the prior art live insertion circuit has problems such as an error caused by fluctuations in power voltage, a short service life of terminals by an overcurrent resulting from a surge current, and noise interference to other package signal lines.